In recent years, Orthogonal Frequency Division Multiplexing (“OFDM” for short) technology has become a mainstream technology of physical layer technology in wireless communication because it is capable of effectively resisting multipath interference and narrowband interference and has a high spectral efficiency. Compared with Code Division Multiple Access (“CDMA” for short) technology of the 3rd generation, the technology of Orthogonal Frequency Division Multiple Access (“OFDMA” for short) plus Multiple Input Multiple Output (“MIMO” for short) has technical advantages of its nature, and is more suitable for broadband mobile communication system and is generally accepted as one of the core technologies of the next generation mobile communication system. IEEE 802.16e standard, which uses OFDMA technology as the core technology of physical layer and meanwhile takes account of mobility and wideband characteristics, is a powerful competitor of the next generation mobile communication standard.
In order to support co-frequency networking, the 802.16 standard divides one carrier frequency into a plurality of segments, each segment includes a subcarrier set, in which the subcarriers do not overlap with each other, in the carrier frequency. Due to the orthogonality of the OFDM subcarriers, there is no co-frequency interference between adjacent cells using different segments of the same carrier frequency. The precondition of such performance is that the division rule for the subcarriers of each segment must be consistent, i.e., the permutation modes of the subcarriers must be consistent. In general, only when each segment uses, by default, a subcarrier permutation zone of the Partial Usage of Sub-Channels (“PUSC” for short), the above condition can be easily satisfied.
However, in order to support various optional enhancing technologies in the OFDMA physical layer, the 802.16 standard establishes a plurality of permutation methods, including PUSC, Full Usage of Sub-Channels (“FUSC” for short), Full Sub-Channel PUSC, optional FUSC, Adjacent Subcarrier Allocation (“Band AMC” for short), Tile Usage of Sub-Channels-1 (“TUSC1” for short), and Tile Usage of Sub-Channels-2 (“TUSC2” for short). As shown in FIG. 1, these permutation methods can simultaneously appear in one frame and be divided by permutation zones.
As shown in Table 1 and Table 2, respectively, according to the description of 802.16 protocol, in a downlink subframe, the conversion between downlink zones is indicated by a Space Time Coding Downlink Zone Information Element (“STC_DL_ZONE_IE” for short) or an Adaptive Antenna System Downlink Information Element (“AAS_DL_IE” for short) in a Downlink Map (“DL_Map” for short). The OFDMA symbol offset in the above message is an 8-bit field that is used to denote the start position of the zone. It can be seen which zones are in each frame and the OFDMA symbol offset of each zone can be dynamically adjusted in each frame according to a certain strategy.
TABLE 1STC_DL_ZONE_IE Format FragmentSizeSyntax(bit)AnnotationSTC_DL_ZONE_IE( ){——Extended DIUC4STC/DL_ZONE_SWITCH = 0x01Length4Length = 0x04OFDMA symbol offset8Denotes the start of thezone(counting from the framepreamble and starting from 0)Permutation20b00 = PUSC permutation0b01 = FUSC permutation0b10 = Optional FUSCpermutation0b11 = Optional adjacentsubcarrier permutation. . .}
TABLE 2AAS_DL_IE Format FragmentSizeSyntax(bit)AnnotationAAS_DL_IE( ){——Extended4AAS = 0x02DIUCLength4Length = 0x03OFDMA symbol8Denotes the start of the zone(countingoffsetfrom the frame preamble and starting from0)Permutation20b00 = PUSC permutation0b01 = FUSC permutation0b10 = Optional FUSC permutation0b11 = Optional adjacent subcarrierpermutation. . .}
If adjacent segments use different permutation modes at the same time, since different permutations correspond to different subcarrier permutation modes, it will inevitably result in that a part of the subcarriers in the used subcarrier sets conflict with each other, which leads to interference between signals on the subcarriers, i.e., the co-frequency interference in a macro sense. Next, the PUSC and Band Adjacent Subcarrier Permutation (“Band AMC” for short) will be taken as examples to explain the situation of mutual conflict between subcarriers of different permutations.
For the PUSC, firstly, a sub-channel is divided into several clusters; each cluster includes 14 consecutive physical subcarriers. Physical clusters are renumbered according to a renumbering sequence so as to form logical clusters. Then, the logical clusters are allocated into sub-channel groups (for 1024 fast Fourier transform (“FFT” for short), the downlink includes 6 sub-channel groups). Data subcarrier mapping is performed according to formula (1):subcarrier(k,s)=Nsubchannels·nk+{ps[nk mod Nsubchannels]+DL_PermBase} mod Nsubchannels  (1)
Wherein, Nsubchannels represents the number of the sub-channels; s represents the serial number of the sub-channel from 0 to Nsubchannels−1; k represents the subcarrier serial number in the sub-channels; nk represents (k+13·s)mod Nsubcarriers; subcarrier(k,s) represents the sequence number of the physical subcarrier corresponding to the k-th subcarrier in the sub-channel s; Ps[j] represents the sequence obtained by rotating the permutation sequence to the left for s times; DL_PermBase is a number from 0 to 31, which equals to the cell identifier (ID_Cell) corresponding to a training sequence (preamble) for the first zone, and is assigned in the IE of the DL_MAP for other zones.
In addition, as shown in FIG. 2, the positions of pilots are denoted according to the positions defined in the cluster. From FIG. 2 it can be seen that the positions of the pilots are different when the cluster is of different odd/even symbols.
Wherein, the mapping relation of the cluster is:
  LogicalCluster  =      {                                                      RenumberingSequence              ⁡                              (                PhysicalCluster                )                                                                                        First                ⁢                                                                  ⁢                DL                ⁢                                                                  ⁢                zone                ⁢                                                                  ⁢                or                            ⁢                                                                                                                                                                                                Use                ⁢                                                                  ⁢                All                ⁢                                                                  ⁢                SC                ⁢                                                                  ⁢                indicator                            =              0                                                                                                                                    in              ⁢                                                          ⁢              STC_DL              ⁢              _Zone              ⁢              _IE                                                                          RenumberingSequence              (                              (                                  PhysicalCluster                  +                                                                          Otherwise                                                                                                13                  *                  DL_PermBase                                )                            ⁢              mod              ⁢                                                          ⁢                              N                clusters                                                                                                                  .      The 802.16e protocol-8.4.6.1.2.1.1 could be referred to for the detailed process.
For Band Adjacent Subcarrier Permutation (Band AMC), with the mode of adjacent subcarrier permutation, data subcarriers and pilot subcarriers are allocated on consecutive physical subcarriers. Such a permutation mode is the same to the uplink/downlink. In adjacent subcarrier permutation, the smallest unit is Bin, one Bin being constructed by 9 physically consecutive subcarriers. For 1024FFT, one symbol includes 96 Bins consecutively arranged according to the sequence of from low physical subcarriers to high physical subcarriers, i.e., from 0 to 95.
It can be seen from the above description that the Band AMC is consecutively arranged according to the subcarriers, while the PUSC is discretely arranged according to the subcarriers. Then, in the time duration of one symbol, there is a situation that the logical data/pilot subcarriers of two modes correspond to the same physical subcarrier (i.e. subcarrier conflict).
As shown in FIG. 3, in the case of 1024FFT, the mode of 3 segments Time Division Duplex (“TDD” for short) 2:1 (the ratio of the length of the downlink subframe to the length of the uplink subframe is 2:1) is considered for networking. In different segments, when the zone permutation modes in the same symbol duration are different, “overlapping” area will appear between the segments.
In FIG. 3, in the overlapping area between the PUSC MIMO Zone in segment0 and the Band AMC Zone in segment1, the situation of data being mapped on the same physical subcarrier may appear.
Suppose that the mode for dividing the 3 segments is:
Segment0 occupies subchannel groups (0, 1) and is constructed by the mode of PUSC+PUSC MIMO+Band AMC. The symbol offset of the PUSC Zone is from 1 to 4; the symbol offset of the PUSC MIMO Zone is from 5 to 14; and the symbol offset of the Band AMC Zone is from 15 to 30. The Band AMC Zone uses logical Band0 to logical Band3.
Segment1 occupies sub-channel groups (2, 3) and is constructed by the mode of PUSC+PUSC MIMO+Band AMC. The symbol offset of the PUSC Zone is from 1 to 4; the symbol offset of the PUSC MIMO Zone is from 5 to 8; and the symbol offset of the Band AMC Zone is from 9 to 30. The Band AMC Zone uses logical Band4 to Band7.
From symbol offset 9 (symbol 10) to symbol offset 14 (symbol 15) in the Segment0 and the Segment1 is the overlapping area between the PUSC MIMO Zone and the Band AMC Zone. Upon calculation, there is the situation of several physical subcarriers overlapping in the overlapping area. The specific number of overlapped subcarriers varies according to parameters such as subcarrier permutation base (Permbase), segment identification (SegmentID). In the condition of PermBase=0, SegmentID=0, the number of the overlapped physical subcarriers is more than 90. When the 3 segments are simultaneously considered, the number of the conflicted physical subcarriers is even greater.
To sum up, in the application scenario of a plurality of zones in OFDMA, if each segment independently dispatches the position and the size of each zone, severe co-frequency interference will occur between adjacent zones.